我的阿里云邮箱给263邮箱发件失败，为什么？

您发送的邮件被退回，相关信息如下：
    发件人： mingcha2005@aliyun.com
    主题：蒋文龙的稿件
    原文：邮件原文包含在附件中。
    时间：Wed, 21 Aug 2013 11:10:32  0800
无法发送到 nmrb2795@263.net
    系统应答：(3)the mta server of 263.net -- mx.263.net(211.150.72.49) reply:550 nmrb2795@263.net:user not exist at 2013-08-21 11:10:32
    退信原因：要求的邮件操作未完成，邮箱不可用（例如，邮箱未找到，或不可访问）。
    解决建议：检查收信人地址是否正确。

Sorry to bounce your email, Information related below:
    Sender: mingcha2005@aliyun.com
    Subject: 蒋文龙的稿件
    Original: Original email has been attached.
    Date: Wed, 21 Aug 2013 11:10:32  0800
Can not deliver to nmrb2795@263.net
    Reply:(3)the mta server of 263.net -- mx.263.net(211.150.72.49) reply:550 nmrb2795@263.net:user not exist at 2013-08-21 11:10:32
    Reason:Requested action not taken: mailbox unavailable (E.g., mailbox not found, no access)
    Solution:Check the email address and try again.